The present invention relates to current transformers for instruments for measuring electric currents and having magnetic cores.
Current transformers for electric current measuring instruments are known per se. Essentially, they consist of a magnetic core with a primary winding carrying the current to be measured and a secondary winding carrying the current which has been matched to the instrument. The ratio of the number of turns in the primary winding to that of the secondary winding determines the transformation ratio u of the current transformer. A large transformation ratio u requires a great number of turns in the secondary winding. For instance, a current transformer with a primary winding consisting of a single turn and carrying 10 A will require a secondary winding of 100 turns to produce a measuring current of 100 mA. A large number of turns usually results in an undesirably large winding capacitance which is detrimental to the precision of the current transformer. Current transformers are sensitive to DC currents. Due to the high initial permeability of the magnetic core material, even a small DC current component will generate an induction current high enough to drive the core into saturation. This gives rise to unacceptable transmission aberrations. In order to reduce sensitivity to DC, current measurement can employ ohmic current dividers in the form of shunts. However, precision measurements using the shunt method are subject to errors due to possible temperature differences between individual current paths as well as to the inductive components of the resistances.